Caption: Animation of gravitational waves radiating from two tightly-orbiting black holes as they merge. The yellow structures illustrate regions of strong curvature of space-time. Orange ripples represent distortions of space-time caused by the rapidly orbiting masses. These distortions spread out and weaken, ultimately becoming gravitational waves (purple). Gravity is the distortion of space-time by mass, and changes in this distortion travel in waves at the speed of light, as predicted in Einstein's theory of general relativity. The effect is most pronounced where extremely massive objects are subject to extremely high acceleration, for instance where black holes or neutron stars are in a close orbit. In 2016 it was announced that in September 2015 gravitational waves had been detected for the first time. They emanated from the collision of two black holes, of 36 and 29 solar masses, some 1.3 billion light years away. The waves were extremely faint by the time they arrived at Earth, where they were detected by the two LIGO detectors in the USA.